Look around you. Chances are there’s more concrete than greenery in sight, that you find yourself somewhere within a sprawling warren of roadways, bridges, and buildings rather than a forest filled with trees. Concrete envelops us. In many places, the built environment has swallowed up the natural one.
When we think of climate change, what’s probably called to mind are the carbon dioxide emissions from cars, coal fired power plants, and even older HVAC systems that warrant exploring new, eco-friendly approaches, like sustainable heating and cooling, to reduce the environmental footprint of everyday appliances.
It turns out concrete is also a significant contributor.
Every year we mix more than 4 billion tons of cement. That’s about half a ton per person. Put another way, each year humans produce enough cement to build 11,000 Empire State Buildings. In 2016, worldwide cement production spewed 2.2 billion tons of carbon dioxide into the Earth’s atmosphere, accounting for 8% of the globe’s total CO2 emissions. It might not seem like all that much, but this quote from a recent BBC article puts the number into perspective.
“If the cement industry were a country,” the article reads, “it would be the third largest emitter in the world—behind China and the US.”
Annual emissions from concrete production will need to fall by at least 16% between now and 2030 if we are to meet even the modest goals set by the Paris Climate Agreement. It’s ironic, then, that worldwide demand for concrete is expected to rise at the exact same time we need to rapidly curb its carbon footprint. A report by Chatham House estimates that cement production is currently on course to balloon to 5 billion tons per year over the next three decades. This explosion will be brought on by increased economic growth and urbanization—specifically in developing regions like Southeast Asia and sub-Saharan Africa.
One way we can rein in concrete and cement related carbon emissions is to start using new building materials, preferably something that’s just as strong and versatile as concrete but significantly less destructive to the living world. One of these materials is something called green concrete.
The Early History of Concrete
Before we dive in, we need to clear up a common misconception: Cement and concrete are not the same thing. The terms are often used interchangeably but they actually refer to two different, albeit closely related, things. Concrete is an amalgam of ingredients, one of which is a binding paste known as (you guessed it) cement. About 10 to 15% of the mixture that comprises concrete is made of cement, while the rest is water and an aggregate of rock, sand, or gravel.
Concrete might seem like a newer, more futuristic building material compared to wood and stone but it’s actually quite old. Humans have used some version of concrete in construction since at least as far back as 9,000 years ago, as evidenced by remnants of concrete flooring found in the ancient city of Yiftah El, west of Jerusalem. Recent research has even revealed that the Egyptians likely built the pyramids out of an early type of limestone concrete poured into wooden molds rather than assembling them piece by painstaking piece out of massive quarried stone blocks, as we were all taught in grade school. It was the Romans who first made widespread use of concrete as a construction material, using a mixture of limestone, volcanic ash, and seawater that has proven stronger than modern versions. It’s funny to think that if they’d been built with contemporary concrete, the two-thousand-year-old Pantheon and Coliseum would have crumbled within fifty years of their construction.
Pictured above: Limestone concrete. Source: Wiki Commons.
Ancient examples of concrete were certainly different from what we use today, but even what we think of as modern concrete has been around for almost two centuries. In 1824, British bricklayer Joseph Aspdin patented Portland cement, which derives its name from the resemblance to the compacted rock Aspdin observed on the Isle of Portland. This 200-year-old formula of combusted limestone and clay powder serves as the primary ingredient in concrete to this day.
The active component in conventional Portland cement is something called clinker, a grey nodular material that hardens when it interacts with the water in the concrete mix. Clinker is formed when chunks of limestone, marl, seashells, and clay, along with a melange of aluminum, silicates, iron, and other minerals are crushed together and superheated in a rotary kiln to about 3,600 degrees Fahrenheit.
As you can imagine, it’s the clinker combustion phase that requires the most energy and accounts for the bulk (about 90%) of the CO2 generated during the cement production process. In the clinker method, it takes the burning of more than 4 million British Thermal Units (BTUs) of petroleum and coal to create a single ton of cement. This fossil fuel driven process in combination with the associated thermal breakdown of calcium carbonate creates about half a ton of CO2 per ton of cement produced. Just a friendly reminder: we produce about 4 billion tons of cement every year. Once the clinker is ready, it’s then mixed with gypsum, ground into a fine powder, and used as the binding agent in most cement products.
And so, the formula for conventional carbon heavy concrete can be summed up in these six words: no clinker, no cement, no concrete.
What Is Green Concrete?
Green concrete isn’t anything new, but recent advances have brought us closer than ever to creating a building material that’s even stronger than conventional concrete while also being significantly less harmful to the environment.
One thing to know about green concrete is that green concrete is not just one thing. There’s no individual product or trademarked brand name, and no, green concrete is not actually green. Similar to health food store labels like “all natural” or “organic,” green concrete actually refers to a wide assortment of more eco-friendly cement technologies that are either in development or currently available on the market. To fall under this categorical header, the concrete in question must use some sort of recycled waste material as one of its components, be made of a carbon neutral or even carbon capturing type of material, or its production process needs to be far less destructive to the environment. The lifecycle and sustainability of the materials involved are also taken into account when determining which concretes qualify as green.
How Is Green Concrete Made? Types of Green Concrete Explained
There are many different approaches to making green concrete. Let’s take a look at a few of them.
Clinker substitution
If clinker is the problem, then use less clinker. So goes the thinking behind the “clinker substitution” approach. The Chatham House report identifies clinker substitutions as one of the cheapest and most effective short-term ways to curb CO2 emissions during the cement production process. As we’ve already covered, it takes an enormous amount of energy to create clinker, the fundamental ingredient of Portland cement. If we replace some or all of it with other stuff, we could dramatically reduce the carbon footprint of cement production. This other stuff is what experts in the field call supplementary cementitious materials (SCMs). These can be of either natural or industrial origin.
One of the more interesting ways to introduce natural SCMs into the equation is to take a page from the ancient Romans, who used volcanic ash to create an extra strong version of cement. If their version is sturdier and lasts longer than ours, is there any reason why we shouldn’t just copy their recipe? That was the question that a group of MIT researchers working in Kuwait recently asked. Their answer: no reason at all. While they didn’t use seawater, the researchers showed that it takes 16% less energy to build a neighborhood of 26 buildings out of concrete with 50% of the clinker based cement replaced with volcanic ash. Volcanic ash has natural “pozzolanic” properties, which means in its powder form, the ash becomes cement-like when exposed to water. Waste byproducts like fly ash, slag, and silica fume generated by manufacturing processes have been shown to possess similar pozzolanic properties, and are also used as effective clinker substitutions to create alternative forms of low-CO2 high-strength cement alternatives. Some research has shown that 50% mixtures of half fly ash or slag cement can cut CO2 emissions nearly in half.
These replacements show serious promise but is the clinker substitution approach perfect? No. For one thing, it only succeeds at reducing the carbon footprint of cement production rather than eliminating it entirely. Volcanic ash and its industrial SCM alternatives also aren’t exactly in high supply. Plus, low-carbon volcanic ash concrete is only suitable for certain projects, skyscrapers not among them. Another major downside is that fly ash and slag are generated by fossil fuel driven processes, which is exactly what we’re trying to get away from. The safety of fly ash as an alternative to clinker has also been called into question by advocacy groups like the Public Employees for Environmental Responsibility.
Still, companies continue to play with the ratios and create new recipes of clinker substituted cement with fly ash, slag, and other recycled waste materials that show a striking ability to reduce CO2. For their part, the group of MIT researchers following in the Romans’ footsteps envision a world where entire urban centers are built almost entirely out of ash.
Aggregate substitution
Another way to go about fiddling with the recipe of traditional concrete in a greener direction is to replace the aggregate. It’s true that production of cement accounts for the overwhelming bulk of concrete’s carbon load, but cement only makes up about 10 to 15% of concrete. The rest, if you recall, is water and aggregate, the latter of which makes up about 80 to 85% of concrete mix.
Most aggregate is made of gravel and sand extracted from rivers and coastlines. In the ranks of natural resources most consumed by humans, sand is second only to water. We mine more than 50 billion tons of sand every year, most of it for use in concrete. And we’re running out of it fast. We can significantly reduce the damage done to coastal and riparian ecosystems if we scale back the mining of sand and gravel and substitute them with more sustainable alternatives.
Broken glass, rubber tires, discarded plastic, ceramic waste from demolition sites—all of these have successfully been used as aggregate substitutes, each with varying levels of viability. One of the more popular examples of aggregate substitution is something called papercrete. This mixture of concrete still uses some sand as an aggregate but in far smaller proportions, replacing much of it with a 60% slurry of pulped paper. Researchers have also projected that by substituting 10% of sand with finely ground plastic, we could leave as much as 800 million tons of sand untouched.
Another popular route is to use recycled concrete. The process is fairly simple: get some old concrete and crush it into a material that makes a suitable aggregate for a new batch. A big selling point of recycled concrete is that it skips a second environmentally destructive round of sand and gravel extraction.
None of these aggregate substitutions is perfect of course—papercrete doesn’t stand up too well against rain, plastic concrete has a low melting point, and recycled concrete relies on an initial CO2 heavy production phase—but each one is a step in a more sustainable direction.
Bioconcrete and other novel concrete alternatives
So far we’ve talked about ways to use recycled waste materials to tinker with the conventional formula of concrete to create greener alternatives. Now we’re going to talk about what happens when you throw the conventional formula out the window.
BioMason is a North Carolina-based startup that’s developed a way to grow concrete using a technique that cuts Portland cement completely out of the picture. What’s more, it doesn’t emit any CO2 at all. To make their trademarked biocement formula, BioMason’s scientists inject recycled aggregates with living bacillus microbes, triggering the combination of carbon with calcium to create limestone crystals in a way that mirrors how coral and seashells are formed in nature. The entire process unfolds within 24 hours and doesn’t require any heat or burning of fossil fuels. The company’s first commercially available product is a type of tile dubbed biolith, which it claims has the lowest carbon footprint of any tile on the market. Researchers at the University of Colorado Boulder have successfully used the synechococcus bacterium to achieve a similar effect.
The advances of BioMason and the University of Colorado foreshadow what some might consider the holy grail of green concrete technology: self-healing concrete. The idea is that bacteria can be used not only to grow concrete but to stitch up cracks and breakages that occur in concrete over time, a miraculous feat that can actually be pulled off by an enzyme found in the bloodstream. The idea of self-healing concrete is of particular interest to the Defense Advanced Research Projects Agency (DARPA), which launched the Engineered Living Materials program in 2016 to further develop the technology of self-healing building materials.
A major benefit to the microbial method of concrete creation is that it actually absorbs CO2 instead of emitting it, thanks in part to the photosynthetic nature of the cyanobacteria involved in some of the processes. This brings us to a quality of normal concrete that we’ve conveniently neglected to mention until this point, which is that all concrete absorbs some amount of carbon dioxide. Via a natural process called carbonation, roughly a quarter of the CO2 emitted by the cement production process will be absorbed back into existing concrete structures throughout their lifecycle.
This isn’t to say that concrete suddenly gets a pass. After all, its carbon footprint is still gigantic even with carbonation taken into account. However, researchers at Purdue University have found they can nearly double concrete’s natural CO2 absorption by adding titanium dioxide nanoparticles into the mix.
If adopted, these exciting advances could completely transform our relationship to both the natural and built environments by turning concrete’s carbon output on its head. The main hurdle these novel approaches face is what the Chatham House report refers to as “the valley of death”; the phenomenon of promising innovations not surviving the initial stage of their creation. Breaking through this barrier, the report concludes, will require targeted investment in research and development and a coordinated effort to educate the public about the potential for green concrete technology.
“Given the urgency of the challenge,” the Chatham House report states, “and the time taken historically for technology systems to evolve, a considerable push will be needed to get the next generation of low-carbon cements out of the lab and into the market. Not all will succeed, but those that do could have significant decarbonization potential.”
Sustainable Construction: A Future Written in Stone
Climate change is a topic of pressing concern for the construction industry, which accounts for a staggering 39% of total carbon emissions worldwide. Not only is construction a major contributor to climate change, it’s also one of its biggest casualties. The rampant destruction of real estate and infrastructure by record breaking floods, hurricanes, wildfires, tornadoes, and other extreme weather events like the derecho of 2020 will continue to worsen in intensity and frequency in the years ahead. If we do nothing, the cost of damages from climate change within our lifetimes will fall somewhere between $1 trillion to $5 trillion per year, hobbling the worldwide economy to the tune of $23 trillion by 2050.
In this light it’s clear that the widespread adoption of strong eco-friendly materials like green concrete can’t happen soon enough. Thankfully, the problem of concrete’s high CO2 output is beginning to be taken seriously. In 2018, the Global Cement and Concrete Association released a set of sustainability guidelines aimed at reining in the industry’s carbon footprint. The green concrete market share is projected to grow to nearly $38 billion worldwide by 2024. After walking away from the accord during the Trump administration, the United States officially rejoined the Paris Climate Agreement earlier this year. Products like Holcim’s ECOPact and Ceratech’s hydraulic green cement are now widely available, and earlier this year HeidelbergCement, the second largest cement manufacturer on Earth, announced plans to convert one of its facilities into the world’s first carbon neutral cement plant by 2030. Bio and nano concrete technologies face unique challenges of scale but the winds are turning in their favor.
The truth is that climate change is here to stay. If we act now, though, the worst case scenarios can still be averted and we can begin the long work of healing the planet. Some day, we may look around and find ourselves in solar or wind powered cities and neighborhoods made of living green concrete, a blend of recycled materials and photosynthetic bacteria that grows like coral, heals itself when it’s damaged, and soaks up billions of tons of carbon from the atmosphere.
